The term “kettle,” in general, designates a household electrical appliance for domestic use with a tank intended to contain the liquid to be heated. Kettles also include a heating element with a resistive element generally situated at the bottom of the tank. When this resistive element is supplied with electrical current, it is able, via the Joule effect, to heat the liquid contained in the tank.
Electric kettles are generally characterised by the low electrical power which they use, typically less than 3000 W, and by their relatively low capacity, typically less than 2.5 litres. Usually, such electric kettles are used to bring water to boiling point, as quickly as possible and several times each day. Kettles must therefore, like its other mass-market household electrical appliances, be robust, efficient and cheap.
However, kettles known in the prior art are not entirely satisfactory in terms of reliability and/or cost. Firstly, kettles are known, for example, whose lower part incorporates, beneath the metal dish intended to transmit the heat emitted by the resistive track, a complete sub-assembly grouping together all the components necessary for the functions of heating, regulation, safety, connection and control.
For connection to the source of electrical energy, such sub-assemblies generally include electromechanical connectors which are complex parts, and therefore expensive.
Moreover, all these components must be placed at a certain distance from the heating element, which results in a not inconsiderable spatial requirement. In addition, the function of regulation or detection of boiling is often performed by a component known as a “bimetal” which breaks the power supply circuit to the heating element by a difference in thermal expansion between the two metals of which it is made. However, so that the bimetals can play their role, it is necessary to provide a duct capable of bringing them into contact with the steam produced by the boiling of the liquid contained in the tank. Such an arrangement is liable to add significantly to the manufacturing costs, and therefore the cost price of such a kettle. Moreover, the accuracy of bimetal components is relatively limited, in other words, the cessation of heating via these components occurs at varying times.
More recently, this bimetal component was replaced by an electronic thermometric component, that is, a component capable of measuring the rise in temperature during heating, of the negative temperature coefficient (N.T.C.) resistor type or thermostat type. A thermometric component is understood to be a component delivering an electrical signal capable of changing with the temperature of its surroundings.
However, although such electronic components are actually superior to bimetal components in terms of lifespan and accuracy of temperature measurement, the position of such components near to the heating element poses problems in kettles of the prior art. Thus, as such electronic components are capable of withstanding relatively high temperatures, they are positioned close to the resistive track of the heating element, that is, in proximity to a zone favourable to the formation of scale, as is the case for example with the kettle of document FR-A-2 791 857.
In general, the function of the thermometric components mentioned above is to measure a temperature representative of the temperature of the heated liquid, but they do not measure the temperature of the liquid directly. On the contrary, these thermometric components measure the variation in the temperature of the resistive track, which temperature indirectly represents the temperature of the heated liquid, as is the case for the kettle in the document cited above.
In fact, as long as the liquid is heating, the temperature of the resistive track increases. When the liquid boils, that is, when its temperature no longer varies, the dissipation of the calorific energy from the resistive track to the liquid slows down and as a result, the temperature of the resistive track begins to stagnate. It is the detection of this stagnation by the regulation circuit which leads to the control circuit cutting the power supply to the heating element.
However, the layer of scale which progressively forms with each heating cycle on the dish transmitting the heat to the liquid, tends to thermally “insulate” the resistive track from the heated liquid. Consequently, as scale builds up in the kettle, the temperature at the resistive track, that is, the temperature measured by the thermometric components mentioned above, tends to be less representative of the temperature of the heated liquid.
This drift can cause the period of time taken to heat the liquid to boiling point to become longer or shorter, which is not satisfactory for the user. Either the electricity consumption of the kettle will increase because of the excessive heating period, or the liquid no longer reaches the target temperature, for example 100° C. in the case of boiling.
In the worst case, excessive drift leads to the destruction of a safety component associated with the heating element, such as a thermal fuse, and as a result, to the kettle being switched off permanently.
Furthermore, document WO-A-98 54931 discloses a kettle whose thermometric component is arranged approximately at the centre of a carrier plate for the resistive tracks and whose other control components are moved to a handle of the kettle.
The aim of the present invention is therefore to eliminate, or at least severely restrict, errors in measuring the temperature of a heating element as scale builds up on it, and thus to prevent any drift in the regulation of the temperature of the heated liquid.
Besides remedying unreliability, the aim of the present invention is also to remedy the other disadvantages of kettles of the prior art, particularly in terms of ease of production and thus of cost and spatial requirement.